output transformer for the 12 inch Magnavox speaker for the Sixteen, Masterpiece & early Phantoms

[David C. Poland]

The target output transformer is for the 6V6 push-pull amp to a 39 ohm impedance voice coil.

A VTVM shows a resistance of about 30 ohms for this voice coil. The model Sixteen, 14 tube Masterpiece and the 1938-9 Phantoms using 6V6 tubes use the same 12 inch speaker with the same field coil configuration and 39 ohm voice coil.

This Scott Magnavox speaker is model 302  and part number 645   171.

The speaker basket rim says   Spec-1-1148      also says 1416.

The problem is I need an output transformer,  and probably two.

I have had in my hands about 8 of these speakers over the years, and half had an open primary on one half or the other half of the push-pull primary winding.

It appears the transformer ratio of the primary winding to the secondary is about 20:1. Scott documentation (Scott News, Vol 11 No 2, Masterpiece) indicates this transformer should be rated for 15 watts. 

The one universal output transformer I happen to have has a ration of 17:1 and actually sounds ok but is undersized at about 10 watts.

I have done some searching but the closest 15 watt universal examples max out at about 15 ohms impedance.  (Antique Radio Supply  and Radio Daze). Don't see anything at Playthings-of -the-Past.

Suggestions anyone?

[Norman S Braithwaite]

The impedance ratio is the critical characteristic, not the rated primary and secondary impedance.  The impedance ratio is the turns ratio squared.  Therefore, having a turns ratio of 20:1, the impedance ratio is 400:1 (full primary to full secondary).  The original output transformer, having a specified output impedance of 39-ohms, has a primary impedance of approximately 15600-ohms.  If all you can find is a transformer having an output impedance of 15-ohms, look for one having a rated primary impedance of 400 x 15 ohms or approximately 6000-ohms.  So long as it is rated for 15-watts, it will be electrically near equivalent to the original.

Norman

[Thomas Day]

David:

Check out the Hammond transformer catalog. They make a lot of transformers for older tube guitar amps and also a lot of tube type output transformers. 

Thom

[David C. Poland]

Thanks Norman.

So  a "ball park" output transformer with 15 ohm impedance may be close enough. Then, seems worthwhile to order the 15 watt Hammond P-P universal replacement from AES (or Radio Daze) that can provide 15 ohm impedance - about $40.  On AES,  part P-T125E has an interesting product review comment: that a Mr. Morris had room to add some additional turns to the secondary to get closer to 39 ohms impedance he needed. (Radio Daze offers the same part.) Larger dimensions  than the Scott original will be a bit of a challenge, but maybe the additional iron is beneficial. Can not tell from the specs on either site what the turns ratio may be for the 15 ohm secondary.

Another thing I do not grasp:  push-pull transformer primary evaluation - whether to measure from primary center tap to one end, or to measure from end to end to determine the ratio. I thought center tap to either end (or center tap to both ends and average the result). My original frame of reference was a Scott transformer with primary half open  compared to the half secondary that would be connected to the voice coil. Used a Variac applied to half primary and 1) adjusted the Variac to produce 1 volt ac on the secondary and then 2) measured the actual voltage to the primary, then 3) computed the ratio. An additional unnecessary complication to avoid for now: the Scott's original secondary winding is also center tapped - but only half is connected to the voice coil, unless a remote speaker is added.  I suppose I could repeat ratio calculations on another of my same model speaker having a good output transformer (after disconnecting the voice coil) to get end-to-end figures.

From Gary Schneider's PTOP  site awhile back, I did find an informative 4 page  article at this link . http://www.radioremembered.org/outimp.htm               

Took some thought to understand, but it enlightened me about speaker impedance, reflected load, tips on evaluating old
output transformers you might already have, and some basic calculations to determine the needed transformer ratio. I 
recommend this piece to those wanting to understand more about impedance. Learned that the measured voice coil 
resistance  times 1.25 is pretty close to the voice coil impedance. Hence the measured 30 ohms does yield the 39 ohm 
spec of the Scott voice coil. This article got me as far as I got to patch in the universal push-pull transformer I had 
and having a ratio near what I had calculated - and attained proper sound from the Scott Masterpiece on my bench. 
Wish the article was clearer on push-pull transformers, however.

So if I were to add some turns to the Hammond transformer, what gage transformer wire should try? (I have a selection of gages available.)

Maybe I should try to open up the bad Scott output transformer and explore it.

[Norman S Braithwaite]

Dave:

I would want to see the tables before ordering the transformer.  Most of the impedance ratios will be duplicate ratios such as 2500:8 and 5000:16.  An impedance ratio of 6000:15 is electrically similar to 15600:39.  Unless stated or marked otherwise, the primary impedance of push-pull transformers is plate to plate.

Norman

[David C. Poland]

Thomas - Thanks. The Hammond site for classic P-P transformers show the same 15 watt units as AES and Radio Daze, but with more information and a chart of connection combinations. Does not show ratio information.

Norman - Thanks. So I'll repeat my analysis with the Variac and a known good transformer from another Scott speaker. Then calculate plate to plate impedance needed. Then from the Hammond hook up chart, then I would select the lugs closest to the impedance the Scott needs.

I'll try that later this evening.

[palegreenthumb]

Dave, also check out Edcor USA, they have some 15W transformers for PP output.

http://www.edcorusa.com/gxppseries

-Rodney

[David C. Poland]

This is my solution. I modified the 125E push-pull Hammond 15 watt transformer from Radio Daze.

Available universal output transformers max out at 15 or 16 ohms including this transformer.
This 125E universal transformer  ( lug 1 to lug 6) has a turn ratio of 26:1. I needed 21:1 (measured from another good Scott transformer for this Scott speaker, driven by 6V6 tubes in push-pull). The Scott models Sixteen and earlier Phantoms use  the same 12 inch speaker.  The Super XII uses a different speaker because of field coil design, but I suspect the same voice coil and ( being rated for 9 watts), probably the 10 watt Hammond push-pull  transformer could be similarly adapted.
For this particular Hammond transformer I chose, there is room between iron laminations and the core to carefully add a layer of wire. After experimentation, I found 12 feet of #22 gage magnet wire connected to the #6 lug and continuing to the RIGHT of lug 6 adjusted the transformer to about the desired 21:1 turn ratio. The new magnet wire is totally exposed.
Used electrical tape on the metal to protect the wire from abrasion while winding the additional wire around the core. Wind the wire fairly tight and neat, and slip 3 or so inches of spaghetti at the end, so the end of the added wire end can be wedged between lug 1 and lug 2 to become a lug #7. Thus:  lug 1 and the new lug 7 is about the ratio I wanted.
This transformer will not fit into the original bracket which also holds the speaker terminal strip.. So I relocated the original bracket enough so I could mount the new transformer tightly agains the original bracket (after removing the old transformer from the old bracket).
Success - the radio sounds good and the impedance is about right.

I will add a couple photos soon.

[David C. Poland]

Here are the images of the modified Hammond output transformer and of the transformer mounted on the speaker.